All first round series are either 2-2-1 or 2-3. As stated before, it's probably a travel issue, although the extra trip only happens if the series goes to 5 games.
The arenas both seem equally available. The Penguins have 4/22 for a comedian, and both teams have 4/28-5/1 roped off for the circus. It could also be an issue of which schedule will get the most rear ends in seats - maybe WBS is a weekday kind of crowd, while Providence is a weekend crowd. The latter must be true, as the P-Bruins basically don't play any home games outside of Fri/Sun except for a few of Saturdays and a rare holiday weekday or the home closer. Maybe the Bruins don't do weekdays well, and/or maybe they do weekends REALLY well.
Fun little wrinkle: If the AHL selects a 2-3 schedule, the top seed actually can choose to give up home ice advantage in return for first-ice advantage.